Which ancient cults would cut out the hearts of live animals and offer them as sacrifices? The Mishnah (Avodah Zarah 2:3) mentions a concept called עורות לבובין which Maimonides and other commentators explain refers to the pagan practice of making an incision in live animals and removing their hearts for ritual purposes. Do we have any documentation as to which ancient cults were engaged in this practice?
A possible answer:
Rambam on the mishnah states that this was done as part of worship of Baalim. The city of Baalbek, which was originally a Baal center of worship, eventually became a center of Bacchus (Dionysus to the Greeks) worship. On this Richard C. Steiner wrote in the essay "On the Rise and Fall of Canaanite Religion at Baalbek: A Tale of Five Toponyms":
"Residual effects of the cult of Bacchus, with its wine-drinking rituals and competitions can perhaps be seen in a medieval descritption of Baalbek. In the introduction to his geographical treatise (tenth century C.E.), al-Muqaddasi writes: "There are no greater drinkers of wine(s) than the people of Baalbek and Egypt."...It is hard to imagine a closer phonetic match than that between Ba'labakku and Ba'al-Bacchus...Since Hadad-Baal and Bacchus-Dionysus are both fertility gods portrayed (frequently) with bull horns, syncretism between Hadad-Baal and Bacchus-Dionysus is by no means unnatural. Indeed, Julius Wellhausen seems to have viewed this syncretism as self-evident, speaking of "Baal-Dionysus" and "the Baal whom the Greeks identified with Dionysus."..."
It's possible that Rambam was referring to this syncretism between the Baal of Baalbek and Bacchus.
"A metrical lex sacra of a Bacchic association in Smyrna prohibits people from eating the heart and the meat of an animal that has not been sacrificed."
From here it sounds like that indeed the heart was removed prior to sacrificing the animal.
A more detailed study of the text can be seen here. According to that study, there seems to have been a number of heart-related rituals. One may have been "the heart seems to have been cut out of the victim separately, placed on the altar, and sprinkled with fat or blood". Another perhaps was "a sacrifice of a ram and goat to Dionysos Zagreus, where the heart was not eaten, but taken away".
So it's possible that the mishnah was referring to a Bacchian or Dionysian ritual, which the Rambam related to Baalim, a later evolution/syncretism of that type of worship that was still known about circa the Rambam's time.